It is a special feature of gravitational amplitudes that an infinite number of particles has to be exchanged at tree-level in order for the amplitudes to satisfy (REF ). This follows from the fact that such exchanges at tree-level correspond to non-minimal couplings, which are known to lead to violations of Eq. (REF ) [1]}, [2]}, [3]}. The only exception to this rule is general relativity, which corresponds to the minimal self-coupling of the graviton.
